GPS Precise Point Positioning Using IGS Orbit Products

The International GPS Service (IGS) has provided GPS orbit products to the scientific community with increased precision and timeliness. Many users interested in geodetic positioning have adopted the IGS precise orbits to achieve centimeter level accuracy and ensure long-term reference frame stability. Currently, a differential positioning approach that requires the combination of observations from a minimum of two GPS receivers, with at least one occupying a station with known coordinates is commonly used. The user position can then be estimated relative to one or multiple reference stations using differenced carrier phase observations and a baseline or network estimation approach. Differencing observations is a popular way to cancel out common GPS satellite and receiver clock errors. Baseline or network processing is effective in connecting the user position to the coordinates of the reference stations while the precise orbit virtually eliminates the errors introduced by the GPS space segment. This mode of processing has proven to be very effective and has received widespread acceptance. One drawback is the practical constraint imposed by the requirement that simultaneous observations be made at reference stations. The following details a post-processing approach that uses un-differenced dual-frequency pseudorange and carrier phase observations along with IGS precise orbit products, for stand-alone precise geodetic point positioning (static or kinematic) with centimeter precision. This is possible if one takes advantage of the satellite clock estimates available with the satellite coordinates in the IGS precise orbit products and models systematic effects that cause centimeter variations in the satellite to user range. This paper will describe the approach, summarize the adjustment procedure and specify the earth and space based models that must be implemented to achieve centimeter level positioning in static mode. Furthermore, station tropospheric zenith path delays with centimetre precision and GPS receiver clock estimates precise to 0.1 nanosecond are also obtained.